The present invention relates to an input interface unit for programmable logic controller which allows the currents to flow to only limited input terminals required in response to a predetermined timing for feeding input data into a central control unit (CPU).
Conventionally, in the input interface unit for programmable logic controller, its circuit is arranged so as to allow the current always to flow through external switches while these external switches (with or without contacts) connected to the input terminals are being closed, so that the capacity of its power supply circuit must be sufficiently large for delivering current even when all switches are closed at the same time. Therefore, if the number of input terminals increases, the capacity of the power supply circuit must be also increased accordingly, so that such an unit is extremely inconvenient in that compactness and reduction in weight of the unit becomes difficult or an external power supply equipment becomes necessary for the input interface unit. Also, if such an external power supply equipment is connected, isolation with photo couplers is needed for cutting off the noises from the outside but this will increase the cost and require more space for mounting the circuits since one coupler is needed for each input terminal.